Petroleum fluids typically contain carbon dioxide and hydrogen sulfide, as well as various hydrocarbons, such as methane, ethane, propane, normal butane and isobutane. Water, present as a vapor and/or as a liquid phase, is also typically found mixed in varying amounts with such hydrocarbons. Under conditions of elevated pressure and reduced temperature, clathrate hydrates can form when such petroleum fluids contain water. Clathrate hydrates are water crystals which form a cage-like structure around "guest" molecules such as hydrate-forming hydrocarbons or other gases. Some hydrate-forming hydrocarbons include, but are not limited to, methane, ethane, propane, isobutane, butane, neopentane, ethylene, propylene, isobutylene, cyclopropane, cyclobutane, cyclopentane, cyclohexane, and benzene. Other gases which may form hydrates include, but are not limited to, oxygen, nitrogen, hydrogen sulfide, carbon dioxide, sulfur dioxide, and chlorine.
Gas hydrate crystals or gas hydrates are a class of clathrate hydrates of particular interest to the petroleum industry because of the pipeline blockages that they can produce during the production and/or transport of natural gas and other petroleum fluids. For example, at a pressure of about 1000 kPa (145 psi), ethane can form gas hydrates at temperatures below 4.degree. C. (39.degree. F.), and at a pressure of 3000 kPa (435 psi), ethane can form gas hydrates at temperatures below 14.degree. C. (57.degree. F.). Such temperatures and pressures are not uncommon for many operating environments where natural gas and other petroleum fluids are produced and transported.
As gas hydrates agglomerate, they can produce hydrate blockages in the pipe or conduit used to produce and/or transport natural gas or other petroleum fluids. The formation of such hydrate blockages can lead to a shutdown in production and thus substantial financial losses. Furthermore, restarting a shutdown facility, particularly an offshore production or transport facility, can be difficult because significant amounts of time, energy, and materials, as well as various engineering adjustments, are often required to safely remove the hydrate blockage.
A variety of measures have been used by the oil and gas industry to prevent the formation of hydrate blockages in oil or gas streams. Such measures include maintaining the temperature and/or pressure outside hydrate formation conditions and introducing an antifreeze such as methanol, ethanol, propanol, or ethylene glycol. From an engineering standpoint, maintaining temperature and/or pressure outside hydrate formation conditions often requires design and equipment modifications, such as insulated or jacketed piping. Such modifications are costly to implement and maintain. The amount of antifreeze required to prevent hydrate blockages is typically between 10% to 30% by weight of the water present in the oil or gas stream. Consequently, several thousand gallons per day of such antifreeze can be required. Such quantities present handling, storage, recovery, and potential toxicity issues. Moreover, these solvents are difficult to completely recover from the production or transportation stream.
Consequently, there is a need for a gas hydrate inhibitor that can be conveniently mixed at low concentrations in the produced or transported petroleum fluids. Such an inhibitor should reduce the rate of nucleation, growth, and/or agglomeration of gas hydrate crystals in a petroleum fluid stream and thereby inhibit the formation of a hydrate blockage in the pipe conveying the petroleum fluid stream. As discussed more fully below, the inhibitors of this invention can effectively treat a petroleum fluid having a water phase, or a petroleum fluid containing water vapor that may condense to form a water phase, depending upon the operating environment.
The use of polymeric inhibitors has been proposed, however, these materials have a tendency to precipitate out of solution at higher temperatures. This is an undesirable characteristic, since the inhibitor must stay in solution under a wide range of temperatures to be most effective. The surfactant monomers described herein yield homopolymers and copolymers with good inhibition properties as well as better solubility at higher temperatures.